I just started the 5x5 and this is my second week. I'm not sure exactly how long to go for but I guess you would just go until you can't add weight anymore then you would switch up the routine to get over the hump. But i'm no professional so thats just my thinking behind it.
i think its better to work through it, very stuctured and good for gains IMO. i plateaued on a few lifts but was able to move up in about 2 weeks not them
Unless I mis-understood, according to Needsize, stay on it all the time.
When you plateau, switch to 5X3 then back down to 5X5, then maybe even to a 3X5, etc.
Myself however after doing it for a year and half or more I would say taking a couple of weeks off here and there, then re-starting the program would work well too. I guess it really depends on you.
Thats the way I've always done it, but this year I've started switching every 8-10 weeks between a version of the 5x5, and a lower volume high intensity approach. The changes to my training do seem to be helping